FIGS. 1, 2, and 3 show that alternating current semiconductor switches (hereinafter referred to as alternating current switches) that controllably switch on and switch off an alternating current signal by control signals. These alternating current switches employ two high-voltage FETs to controllably switch on and switch off the alternating current signal applied to first and second terminals 11 and 12.
An alternating current switch shown in FIG. 1 is comprised of a normally off type MOS FET Q11 (hereinafter referred to as FET Q11) and a normally off type MOS FET Q12 (hereinafter referred to as FET Q12), which are connected in series in opposite direction between the first and second terminals 11 and 12. With an alternating current switch shown in FIG. 2, a normally off type FET Q13 and a normally off type FET Q14 are connected in series in opposite direction and the connection between a drain and a source is opposite to that shown in FIG. 1.
With the alternating current switch shown in FIG. 1, if a first gate signal lies at a positive voltage and is applied from the gate terminal G1t to a gate G1 of the FET Q11 while a second gate signal lies at a positive voltage and is applied from the gate terminal G2t to a gate G2 of the FET Q12, both the FET Q11 and the FET Q12 are turned on. Therefore, during a period wherein the first and second gate signals lie at the positive voltage, a current flows from the first terminal 11 to the second terminal 12 when the first terminal 11 is applied with the positive voltage whereas a current flows from the second terminal 12 to the first terminal 11 when the second terminal 12 is applied with the positive voltage.
Next, if the first and second gate signals lie at zero voltage and are applied to the gates of the FET Q11 and the FET Q12, both the FET Q11 and the FET Q12 are turned off. For this reason, no current flows through the alternating current switch.
Also, the alternating current switch shown in FIG. 2 operates in the same manner as that shown in FIG. 1.
An alternating current switch shown in FIG. 3 is comprised of a first series circuit, composed of a diode D11 and a normally off type FET Q15, and a second series circuit, composed of a diode D12 and a normally off type FET Q16, which are connected in parallel between the first and second terminals 11, 12. An anode of the diode D11 is connected to the first terminal 11 and an anode of the diode 12 is connected to the second terminal 12.
With the alternating current switch shown in FIG. 3, if a first gate signal takes a positive voltage and is applied from a gate terminal G1t to a gate G1 of the FET Q15 while a second gate signal takes a positive voltage and is applied from a gate terminal G2t to a gate G2 of the FET Q16, both the FETs Q15 and Q16 are turned on. For this reason, a current flows in a path expressed as “First Terminal 11→Diode D11→FET Q15→Second Terminal 12”. That is, during a period in which the first and second gate signals take a positive voltage, a current flows from the first terminal 11 to the second terminal 12 when the second terminal 12 is applied with a positive voltage. Also, when the second terminal 12 is applied with a positive voltage, a current flows in a path expressed as “Second Terminal 12→Diode D12→FET Q16→First Terminal 11”. That is, the current flows from the second terminal 12 to the first terminal 11.
Next, if the first and second gate signals take zero voltage and are applied to the gates of the FETs Q15, Q16, both the FETs Q15 and Q16 are turned off. Therefore, no current flows through the alternating current switch.
However, with the alternating current switches shown in FIGS. 1 and 2, since the high-voltage elements with high on-resistance are connected in series, on-resistance of the alternating current semiconductor switch remarkably increases, resulting in an increase in losses. Further, with the alternating current switch shown in FIG. 3, the number of component parts increases with a resultant increase in costs.
In the meanwhile, although the FET of the semiconductor, made of compound such as SiC and GaN or the like, has a high withstand voltage but low on-resistance, and is highly suited to a heavy-power switch, only a so-called normally on type FET (an FET through which drain current flows when a gate signal falls at a zero potential) can be manufactured. With such a normally on type FET, no gate signal exists during a time interval in which a power supply is turned on and, hence, a drain current is caused to flow with resultant damages to the normally on type FET, resulting in an extremely difficult usage. For this reason, there has been a need for development of an FET in which no current flows even in the presence of a gate signal at a zero potential.
Therefore, as shown in FIG. 4, proposal has been made in the past to use a direct current switch wherein a normally on type FET Q18, composed of SiC with a high voltage, and a normally off type FET Q17 with a low voltage and low on-resistance are connected in cascade between the first and second terminals 11 and 12 as disclosed in Japanese Patent Application Laid-Open NO. 5-75110. This direct current switch is configured to be of a high voltage with low on-resistance and a direct current signal is applied across the first and second terminals 11 and 12.
With the direct current switch shown in FIG. 4, if a gate G1 of the FET Q17 is applied with a voltage greater than a threshold value, the FET Q17 is turned on and the FTE Q18 is also turned on. Further, if the gate G1 of the FET Q17 is applied with a voltage less than the threshold value, the FET Q17 is turned off and the FTE Q18 is also turned off. That is, the direct current switch is turned on or turned off when the voltage is applied to the gate G1 of the FET Q17 and the FET Q17 can serve to operate as if it were a single FET with a high withstand voltage.
However, the direct current switch, shown in FIG. 4, cannot be used for alternating current switch. For this reason, the alternating current switch has been realized using the circuits shown in FIGS. 5 and 6.
The alternating current switch, shown in FIG. 5, has a circuit structure in which the direct current switch, shown in FIG. 4, is applied to the alternating current switch shown in FIG. 3. The FETs Q19, Q21, shown in FIG. 5, correspond to the FET Q15 shown in FIG. 3, and the FETs Q20, Q22, shown in FIG. 5, correspond to the FET Q16 shown in FIG. 3, operating in the same manner as those shown in FIGS. 3 and 4, respectively.
The alternating current switch, shown in FIG. 6, has a circuit structure in which the direct current switch, shown in FIG. 4, is applied to the alternating current switch shown in FIG. 1. The FETs Q25, Q26, shown in FIG. 6, corresponds to the FET Q11, shown in FIG. 1, and the FETs Q23, Q24, shown in FIG. 6, corresponds to the FET Q12 shown in FIG. 3, operating in the same manner as those shown in FIGS. 3 and 4, respectively.